Absolute half-cell potential: A simple direct measurement

The concept of the absolute half-cell potential is discussed from a simple thermodynamic point of view and defined as -{Δ}G/ne for the half-cell reaction, treating the electron like other individual chemical species. With the standard reference state of the electron taken as the free electron at rest at infinity, the absolute half-cell potential is simply the Fermi level of the electrons in the half-cell lead wire. This potential can be simply measured with a Kelvin probe by determining the vacuum potential level over the electrolyte relative to the Fermi level of the half-cell lead. Since the probe cannot determine potentials absolutely, it must be calibrated using a material whose work function is known. Using this procedure we obtain for the standard hydrogen half-cell, E_abs(SHE)=4.456±0.025 eV, assuming our Hg as used in calibration has a work function of 4.495 eV.